First Report of Alfalfa Anthracnose Caused by Colletotrichum americae-borealis in Xinjiang, China

Alfalfa ( L.) is widely planted in the world as one of the most important leguminous forage crops, and it is also the first choice of forage crops for animal feed in Xinjiang. In June 2018, alfalfa plants with typical anthracnose symptoms were observed in 75% of alfalfa fields in Hutubi County, Xinjiang, China. The disease usually occurred in alfalfa fields that had been planted for more than 2 years and was distributed in patches in the field. The incidence rate ranged from 7.5% to 53%, and the fatality rate ranged from 0 to 3%. Greater incidence was observed in fields with older stands. At the early stage of disease, pale brown prismatic or oval sunken lesions with dark brown to black edges were observed at the base of the stem of alfalfa plants. As the symptoms progressed, lesions on stems turned necrotic, and the center of the lesion became gray-white with black dots. In severe cases, the lesion expanded around the stem, causing the upper part of the stem to break off, or wilt and die. Twenty plant stem sections with typical symptoms were sampled and surface-sterilized with 75% ethanol for 30 s and 1% NaClO for 1 min, rinsed in sterilized distilled water, dried on sterilized filter paper for 45 s, placed on potato dextrose agar (PDA), and incubated in the dark at 25°C for 7 days. A fungus was frequently isolated from the surface-sterilized segments, and the colonies of this fungus were white and flat at first, and later the center of colonies became pale brown with black microsclerotia (2.0~3.2 mm. n = 30) and white or brown acervuli (1.0~1.8 mm. n = 30). A large number of conidia and setae spread from ruptured microsclerotia under microscopy. Conidia (n = 40) were hyaline, smooth-walled, straight, aseptate, cylindrical to fusoid, both tips acute to round, 13.7 to 19.5×3.0 to 4.5 μm . Setae (n = 30) were dark brown to black, smooth-walled, 3~6 septate, straight or slightly curved, 66.9~185.1 μm long, tip round and base swollen, 3.9~5.2 μm width. Sometimes setae formed directly on hyphae or brown acervuli. Colony and conidia morphology were similar to the description of (Damm et al. 2014; Lyu et al. 2020). DNA was extracted from fresh mycelia of three representative isolates (R11, R12 and R13) and the ITS, ACT, CHS-1 and HIS3 genes of three isolates were amplified and sequenced using the primers described previously by Damm et al. (2014). The sequences of three isolates were identical, and twelve aligned sequences from three representative isolates wer